1. Field of the Invention
The present invention relates to a media width detecting apparatus provided in a printer, for example, a serial impact dot matrix (SIDM) printer, so as to detect the width of the print media.
2. Description of Related Art
In conventional art, for example a SIDM printer, there is provided is an apparatus having a function of detecting media width as well as having a function of canceling the skew of print media. In this kind of apparatus, an inserted print medium is conveyed by rollers with comparatively small coefficients of friction and the print medium is pressed to a shutter. The entire front edge portion of the print medium is then pressed to the shutter, so as to cancel skew.
Moreover, in order to detect the position of every portion of the print medium inserted in the apparatus, that is, the left end position and the right end position; sheet width sensors are provided on a carriage conducting a spacing motion. This allows the left end position and the right end position of the print medium are detected by the sheet width sensor when the print medium is inserted with the moving carriage.
The point of detecting the sheet width of the print medium is set at a position of ¼ inch, for example, apart from the front edge of the print medium. Optical sensors of reflection type are used for sheet width detecting sensors. Light is then emitted to surface of the print medium and, reflected light from medium is received by the sensor, thus detecting whether a medium exists or not. This kind of media width detecting apparatus is disclosed in, for example, JP11-208928.
However, in the conventional apparatus mentioned above, sheet width sensors provided on a carriage, move across the range of the whole width of the media conveying path, so as to detect side end portions of a print medium. Therefore, if there is something, for example dust or a piece of sheet etc., on the platen which confronts the carriage; the dust or piece of sheet etc., can be detected as print medium by mistake. In this occasion, medium width is not detected properly, and the print starting position can be outside of the medium. And therefore printing cannot be performed properly. Moreover, printing is performed directly on the platen thus damaging the platen and the print head.
Moreover, as mentioned above, the conventional apparatus detects sheet width at a prescribed position (for example, a position of ¼ inch apart from the front end) by using optical sensors of a reflection type as sheet width detecting sensors. Such sensors can prove problematic when the print medium contains dark pre-printed matter. As a result, the sheet width sensors can detect the end portion of a pre-printed portion of the print medium as the end position of the print medium itself.